1. Field of the Invention
The present invention relates, generally, to communication systems and processes which use radio frequency (RF) transmitters and receivers, and, in particular embodiments, to multi-band systems and processes which share frequency sources and filters to minimize size, weight, complexity, power consumption, and cost.
2. Description of Related Art
It has become increasingly important to minimize the size, weight, complexity, power consumption, and cost of various electronic devices, especially personal communication devices such as cellular telephones, personal pagers, cordless telephones, and the like. One way to minimize such characteristics is to minimize the number of components and functions required in the electronic device, or to perform multiple functions using the same components. However, personal communication devices such as cellular telephones often require complex circuitry with a number of power-inefficient components for performing particular functions. This is especially true in modern cellular communications, where several different communication standards are employed worldwide, and cellular telephones with the flexibility to operate under multiple communications standards are highly desirable from a consumer and manufacturing perspective.
For example, GSM900 (Global System for Mobile 900) is a digital cellular standard operating in the 900 MHz frequency band that is currently used in Europe and Asia. DCS1800 is another digital cellular standard based on GSM technology, operating in the 1800 MHz frequency band and also currently used in Europe and Asia. The United States uses PCS1900, a third digital cellular standard similar to DCS1800, but operating in the 1900 MHz band. Multi-band cellular telephones capable of operating under all of these standards afford consumers widespread applicability and allow manufacturers to benefit from the cost-efficiency of a common design.
However, multi-band cellular telephones present a number of design challenges. Conventional single-band transmitters typically require two separate frequencies, a fixed intermediate frequency (IF) for modulation and a tunable radio frequency (RF) for upconversion. Conventional single-band receivers also typically require two separate frequencies, a tunable RF for downconversion and a fixed IF for demodulation. Without the sharing of frequency sources, a single-band cellular telephone may require as many as four different frequency sources. Multi-band cellular telephones exacerbate the problem because the modulation, upconversion, downconversion, and demodulation processes for each band may require different frequencies. Furthermore, the different frequencies employed by each band may require different filters for the transmit and receive function of each band. Size, weight, complexity, power consumption, and cost savings may be achieved by designing multi-band transmitters and receivers to share the same frequency sources and filters.
Therefore, it is an object of embodiments of the present invention to provide a system and process for a multi-band communication unit that shares frequency sources and filters between bands in the transmitters and in the receivers to minimize size, weight, complexity, power consumption, and cost.
These and other objects are accomplished according to a communication system for communicating RF signals at any one of a plurality of communication standards through a common antenna. Each communication standard has a unique transmit band, receive band, and guard band between the transmit and receive bands, and all communication standards have a common data rate. The communication system comprises a transmitting unit, a receiving unit, an IF LO frequency generator, an RF LO frequency generator, a reference source, and an antenna.
The transmitting unit has a transmit RF information signal output, a modulator for transforming a transmit baseband information signal into a modulated transmit IF information signal, and an upconverter for upconverting the transmit IF information signal into a transmit RF information signal. The receiving unit has a receive RF information signal input, a first downconverter for downconverting a receive RF information signal to a first receive IF information signal, a second downconverter for downconverting the first receive IF information signal to a second receive IF information signal, and an analog-to-digital converter for converting the second receive IF information signal to a receive digital information signal.
The IF LO frequency generator is coupled to the transmitting and receiving units and generates a transmit IF LO and receive-IF LO. The RF LO frequency generator is also coupled to the transmitting and receiving units and generates a transmit RF LO and receive RF LO. The IF LO frequency generator and the RF LO frequency generator are coupled and phase-locked to the reference source. The antenna is coupled to the transmit RF information signal output and the receive RF information signal input for transmitting and receiving RF information signals.
The communication system further includes a second filter coupled between the second mixer and the analog-to-digital converter for filtering the second receive IF information signal,.a first filter coupled between the first mixer and the second mixer for filtering the first receive IF information signal, and a transmit IF filter coupled between the modulator and the translation loop.
In order to share frequency sources and filters between bands in the transmitters and in the receivers, the frequencies of processing clocks, sample rates, and frequency generators and sources, the carrier frequencies of the first and second receive IF information signals, and the center frequencies and bandwidths of the filters are governed by a set of design preferences. These design preferences include:
selecting the processing clock based on the data rate of a given communication standard,
selecting the reference frequency and the analog-to-digital converter sample rate based on the requirement that the analog-to-digital converter have a low phase jitter sample clock,
selecting filters, the analog-to-digital converter sample rate, and the frequency of the first and second receive IF information signals to avoid harmonics of the processing clock,
selecting the receive RF LO to avoid mixer spurious responses, selecting transmit IF LO, receive IF LO, transmit RF LO and receive RF LO to minimize tuning and settling times of the phase-locked loops in the IF LO frequency generator and the RF LO frequency generator,
selecting transmit IF LO and transmit RF LO to minimize zero-crossing spurs, and
selecting filters to minimize 4xc3x97 crossing spurs. Embodiments of the present invention include systems and processes for shared resource multi-band communication units whose frequency generators and sources, carrier frequencies of the first and second receive IF information signals, and center frequencies and bandwidths of the filters satisfy these design preferences.
These and other objects, features, and advantages of embodiments of the invention will be apparent to those skilled in the art from the following detailed description of embodiments of the invention, when read with the drawings and appended claims.